The manufacturing of boards made from wood materials is based essentially on the pressing of hackled wood pieces, in particular of wood fibers and/or wood chips. For example, a chip board consists of small wood chips with different thickness, which are pressed together with a binder and under application of high pressure to form boards. Wood fiber boards are produced from wooden fiber with or without an additional bonding agent.
Before being pressed to boards, the hackled wood pieces have to be dried. This is usually done in so called drum dryers, wherein the goods to be dried respectively the bulk goods are moved in a heated, rotating tube. During the drying also gaseous wood contents are freed in addition to water vapor, which must not be released to the environment since they are considered as pollutants. The drying vapors are further contaminated with fine particulate matter. For this reasons, the drying vapors have to be cleaned before they can be released to the environment. This is achieved usually by dust removal, filtering and/or a burn-out in the burner of the dryer. To reduce the costs for this treatment of the drying gases and in particular to reduce the additionally necessary energy consumption, different methods and apparatuses are suggested, which enable a more economic process by guiding the drying gases in a circuit and subjecting the same to an indirect heating via a burner.
The European patent application EP 0 459 603 A1 describes for example a drying of wood fibers in a drum dryer, wherein the drying vapors exciting the dryer are led back in a circuit to the dryer and are heated indirectly through the heating gas produced by the burner until they reach the temperatures necessary for drying the wood chips. A part of the drying vapor is removed from this circuit and guided to the combustion chamber. The exhaust gases from the combustion chamber, which are used to heat-up the drying gases via a heat-exchanger, are cleaned with a filter, before they are released to the environment.
The European patent application EP 0 457 203 A1 describes also a drying method among others for wood chips, wherein the drying gases are indirectly heated by a heat-exchanger and wherein the heat-exchanger is energized with the exhaust gases of a combustion chamber. A part of the drying vapors is continuously removed from the dryer and fed to a condenser wherein the water contend is condensed and wherein the non-condensable gases are led as combustion air into the combustion chamber.
With these methods the temperatures in the combustion chamber have to be kept sufficiently high, to assure the burning off of any pollutants. These high temperatures put a strain to the elements of the heat-exchanger so that the lifetime thereof is reduced. For this reason, the European patent application EP 0 714 006 suggests a drying method, wherein a second heat-exchanger is arranged before the first heat-exchanger in order to reduce the thermal strain of the material.
During the drying process in the circuit constantly new vapors are produced which are contaminated with pollutants. The circulating drying vapors therefore have to be continuously removed, to achieve a mass balance. This is done for example by removing a part of the drying vapors downstream or upstream of the heat-exchanger and to guide this part as combustion air to the burning chamber. For the control of the flow rate the European patent application EP 0 714 006 A1 suggests for example a valve.